Ethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focused on producing chemicals, including glycols, from renewable feedstocks, such as saccharide-containing feedstock.
The term saccharide-containing feedstock is used herein to mean any composition comprising mono-, di- and/or poly-saccharides derived from plant matter.
For example, US 2011/312050 describes a continuous process for the catalytic generation of polyols from cellulose, in which the cellulose is contacted with hydrogen, water and a catalyst to generate an effluent stream comprising at least one polyol.
CN 102643165 is directed to a catalytic process for reacting saccharides in an aqueous solution with hydrogen in the presence of a catalyst in order to generate polyols.
As with many chemical processes, the reaction product stream in these reactions comprises a number of desired materials as well as diluents, by-products and other undesirable materials. In order to provide a high value process, the desirable product or products must be obtainable from the reaction product stream in high purity with a high percentage recovery of each product and with as low as possible use of energy, chemical components and complex equipment.
In known processes to make glycols, the glycols are usually present at high dilution in a solvent, typically water. The water is usually removed from the glycols by distillation. Subsequent purification of the glycols is then carried out by fractional distillation.
When glycols are produced by hydrogenolysis of saccharide-containing feedstock, a mixture of glycols is produced. The main glycol constituents in the reaction product stream are monoethylene glycol (MEG), 1,2-monopropylene glycol (MPG) and 1,2-butanediol (1,2-BDO).
The separation of these glycols by fractional distillation is problematic due to the similarity in their boiling points, particularly between MEG and 1,2-BDO (197.3° C. and 195 to 196.9° C. respectively).
More particularly, the isolation of a pure MEG overhead stream by fractional distillation from a mixture comprising MEG and 1,2-BDO is made impossible by the formation of an azeotrope between MEG and 1,2-BDO at temperatures ranging from 20 to 250° C. and pressures ranging from 1 mbar to 3.5 bar.
A number of methods have been described in the art as suitable for separating MEG from a mixture of glycols.
US 2012/0184783 discloses several methods for the extraction of glycols from aqueous streams. In particular this document discloses methods for the selective extraction of individual glycols from concentrated mixtures thereof comprising less than 50 wt % water using a hydrophobic solvent mixture. Optionally, at least one of the hydrophobic solvents present in the mixture forms a heteroazeotropic mixture with MEG and, thus, allows further separation of the glycols.
U.S. Pat. No. 4,966,658 discloses azeotrope forming agents that enhance the relative volatility of ethylene glycol to enable its separation from aqueous mixtures of MEG and butanediols by subjecting such mixtures to hetroazeotropic distillation.
U.S. Pat. No. 5,423,955 discloses azeotrope forming agents that enhance the relative volatility of propylene glycol to enable its separation from 1,2-BDO by homoazeotropic distillation.
However, the molar ratio of the azeotrope forming agent to glycols used in the abovementioned cases is high for entrainers with a lower boiling point than the glycols, or the improvement of relative volatilities of MEG over 1,2-BDO is low for entrainers with a higher boiling point than the glycols, making the overall glycol production process expensive and more complicated to run.
Therefore, it would be advantageous to provide an improved method suitable for the recovery of MEG from mixtures comprising MEG and 1,2-BDO wherein the relative volatilities of MEG over 1,2-BDO is high enough and the amount of azeotrope forming agent used is low enough to make the overall glycol production process more economical than processes disclosed in the prior art.